Of the more than 2 million Americans with heart failure (HF), up to 70% have disturbed sleep that worsens the dyspnea, fatigue, and reduced daytime function associated with HF. Exercise improves sleep in healthy people but the effects of exercise have not been tested in patients with HF. A randomized two-group pretest posttest control-experimental group design is proposed to compare the before (pretest) and after (posttest) effects of 16 weeks of no regular walking (control group) with 16 weeks of regular walking at least 5 times a week for up to 30 minutes (experimental group), on cardiac function and sleep in 152 subjects with moderately severe, stable heart failure. Aim 1: Compare physiologic (peak oxygen utilization, nocturnal desaturations, and heart rate variability [HRV]) and self-reported (dyspnea/fatigue, activities of daily living) measures of cardiac function for HF subjects who do or do not walk regularly for 16 weeks. Aim 2: Compare somnographic (sleep fragmentation, slow wave sleep, sleep efficiency) and self-reported (sleep effectiveness, sleep disturbance, and nap supplementation) sleep of HF subjects who do or do not walk regularly for 16 weeks. Aim 3: Explore whether HF patients with frequent apnea/hypopnea episodes who walk regularly have greater changes in cardiac function and sleep than HF subjects with few apnea/hypopnea episodes who exercise, or the control group of HF patients with frequent or few apnea-hypopnea episodes. Pretest and posttest measures include an exercise tolerance test; two nights of somnographic sleep and oxygen saturation recordings; one 24-hour recording of HRV and activity; and subjective reports of sleep, dyspnea/fatigue, and functional status. One-way analyses of variances (ANOVA) will be used to test for control-experimental group differences in cardiac function (Aim 1) and sleep (Aim 2). Interaction effects will be examined in a two-way ANOVA of group (HF subjects with frequent versus few apnea-hypopnea episodes who do/do not walk regularly) and time (before and after condition) to determine whether HF subjects with frequent apnea-hypopnea episodes who exercise have better outcomes than HF subjects with few apnea-hypopnea episodes who exercise, or the control group (Aim 3).